Photodynamic Therapy: An overview

The use of light in medical treatment is nothing new. In the ancient world, roots of the plant Dorstenia were applied topically on to irritated skin which would clear following a few hours of sunshine. The active ingredient, psoralen, is now the basis of PUVA (psoralen + UVA) therapy used to treat the effects of psoriasis and other skin ailments. Psoralen acts by intercalating between base pairs of DNA and upon UV irradiation, the two double bonds form [2+2] cyclo-adducts with thymine, kinking and destroying the DNA of the replicating cells, which was causing the skin irritations in the first place.

In the early twentieth century, following significant progess in synthetic chemistry of coloured dyes throughout the nineteenth century, two German scientists completed work on the toxic effect of eoisin, a methylene based dye. The effect was only noted with the presence of light, and at a later date the presence of oxygen. Thus the modern day science of photodynamic therapy (PDT) was discovered, consisting of three components: photosensitising dye, light, and oxygen. Despite positive results from trials, the work went relatively unnoticed, and it wasn’t until the 1970s that it really picked up again.

Fundamentals

The basic principles of PDT are relatively easy to consider. A light absorbing dye is applied or injected into the patient and after a time appropriate for maximum uptake into tumour, the affected area is irradiated with light. The dye absorbs the incident light and an electronically excited state is formed. This subsequently generates a reactive oxygen species (ROS) which destroys the tumour. The concept is beautifully simple, and as the dye is non-toxic in the absence of light, does not carry the negative effect of traditional chemotherapies which are much less discriminate in their action. In PDT, only irradiated areas of body tissue will generate activity leading to cell destruction. As mentioned, there are three components to PDT: light, photosensitiser and oxygen. the latter two are considered in turn, below.

Oxygen

The net result of dye irradiation is the generation of reactive oxygen species, and it is generally considered that singlet oxygen 1O2 is the ROS responsible for cell destruction in PDT. 1O2 is formed when oxygen, which in the ground state exists in as a triplet (3O2 ) absorbs energy. According to the MO diagram of the ground state 3O2 shown, the singlet is formed with the pairing of electrons in the LUMO. This is energetically unstable relative to the ground state, as there is a vacant orbital of the same energy available to the paired electron. Hence, 1O2 is energetically higher (thus more reactive) and will return to its triplet ground state. The lifetime of the singlet state in a cell is of the order of 100s of ns, and it has been estimated that it can diffuse less than 50 nm in this time. Therefore, in cells of the order of microns, the action is limited to cellular dimensions.

1O2 is formed by energy transfer from the photoexcited dye. But an alternative is possible. If the photoexcited dye transfers an electron to molecular oxygen, a superoxide anion is formed. Therefore, a crucial aspect of drug development in PDT is the nature of the ROS formed. Electron transfer forming superoxide anion is called a Type I reaction. Energy transfer forming 1O2 is called a Type II reaction. We can distinguish between these by understanding the chemistry behind them. Importantly for us here. Type II is detected because as singlet oxygen returns to its triplet ground state, it emits a small amount of infrared phosphorescence, which can be detected (see figure – the emission maximum is approximately 1270 nm). Type I on the other hand, can be detected by monitoring the redox chemistry of Fe3+ and subsequent formation of hydroxyl radicals (the photo-Fenton mechanism).

Singlet oxygen emission

Photosensitiser

The photosensitiser has several functions. It must locate in the tumour. This involves considering both hydrophilic and lipophilic components in the molecular design that are not covered here. It should also absorb in the far visible region (600 – 800 and preferably 700 – 800 nm). Haemoglobin is a significant component of body tissue, and absorbs strongly in the mid visible region (580 nm). This is obvious when we shine a light through our hand – only red light passes through. Therefore the ideal will be one which absorbs light where the body does not, allowing them to be used deep within body tissue (e.g. liver, pancreas).

As it stands, clinically approved PDT drugs are not yet optimised for longer wavelength light absorption, and hence PDT treatment is currently limited to areas that are easy to expose to a light source: skin, lung, oesophagus, etc. This issue of “penetration depth” was the subject of a recent court case, whereby a doctor justifiably claimed that PDT treatment would not be suitable for treatments such as liver cancer, as the liver was just too big for light to pass through. (The doctor was subsequently acquitted of all charges). The bar chart (Ref 1) shows how photosensitiser absorption capacity affects penetration depth, and this is the focus of current research (below). Penetration doubles once light at longer wavelength than the absorption of haemoglobin is achieved (630 nm) and doubles again at 700 nm. An ideal photosensitiser will therefore absorb between 700 – 800 nm.

Other factors for an ideal photosensitiser include: low toxicity in the absence of light and little post-treatment side affects. One of the most significant side effects is post-treatment light sensitivity, whereby patients have to avoid light for fear that healthy body tissue which have residual amount of photosensitiser present will generate unwanted activity.

Jablonski Diagram for generation of singlet oxygen (ref 1)

From a photochemical point of view, one of the most important dye characteristics is that it will form a high concentration of triplet excited state. Since the mechanism of action is generation of singlet oxygen, a singlet state may not be long-lived enough to allow time for reaction with oxygen (singlet-singlet deactivation via fluorscence or non-radiative means is an allowed process, and therefore very fast). If the triplet forms via intersystem crossing, its deactivation is forbidden, and hence slower, allowing the energy transfer to oxygen to be more competitive. This again provides potential for future research (see below).

Photofrin

Photofrin (R)

The first clinically approved PDT drug was photofrin (R). Photofrin is a porphyrin -based compound and you may wish to examine its structure to identify hydrophilic and lipophilic components alluded to above. It absorbs at 630 nm, which is within the PDT window and has a respectable quantum yield of inter-system crossing of about 25%. However, its absorption is not significant, with a low extinction coefficient. Its worth noting here also that PDT is not limited to cancer based therapy, it has also been used as alternative to antibiotics and for gum disease (There is a good overview article in Chemistry World, ref 2, and some nice pictures for dental treatment at the Periowave site, ref 3)

Current Developments

Absorption spectra of chlorins and bacteriochlorins (Ref 1)

Photofrin’s limitation is primarily its light absorption. To get to a point where PDT can become more versatile, the photosensitiser needs first to absorb in the 700 – 800 nm window (and then subsequently satisfy all other demands re singlet oxygen generation, accumulation in tumours….). Reduction of one (chlorins) and two (bacteriochlorins) of the four pyrroles in porphyrin based compounds have been found to shift the wavelength of absorption to longer wavelengths. In the example shown, the absorption shifts to about 700 nm for chlorin-based molecules and to about 800 nm for bacteriochlorin based molecules. The compound shown has a high extinction coefficient of absorption and good oxygen generation capabilities. More information on these compounds is available in Ref 1.

Intersystem crossing can be enhanced by the heavy atom effect, and this is the subject of another class of boron-based compounds. It was noted for certain sites of substitution of iodine, the singlet oxygen generation capacity increased, attributed to an increased intersystem crossing yield caused by the iodine heavy atoms. More information on these compounds is available in reference 4.

Utilising heavy atom effect to enhance ISC (Ref 4)

Summary

PDT provides huge potential in treatment of cancerous tumours and a range of other antibiotic treatments. It has been called a very selective surgeon’s knife thanks to its ability to isolate the affected area for treatment with little collateral damage. At the core of future developments of PDT is an understanding of the photochemistry at its heart, and now a century after the first PDT action was discovered, it looks like it has a positive future.

References

1. (Primary reference for this article) Chem Soc Rev, 2011, 40, 340: Sections A, B, C.1, C.4, D, F.2, F.3

2. Chemistry World, 2012, April, 52, see also Education in Chemistry, 2004, May, 71.

3. Periowave blog (accessed December 2012)

3. Chem Soc Rev, 2013, 42, 77: pages 77 – 81.

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Avatar and Photochemistry: Chemiluminescence

Photochemistry for an Oscar? In the movie Avatar, it plays a central role, although I must admit I didn’t see it listed in the credits… The movie is set on a planet called Pandora, and at night time, the forests of Pandora light up to give some really beautiful cinema, all in 3D! This article explains the glow in the trees, insects, inhabitants and just about everything else on Pandora at night time. Back on Earth, we’re familiar with this glow too!

When some chemicals react, they can give off or require huge amounts of energy in doing so, as existing bonds are ripped apart and new ones form. This energy is in the form of heat – reactions can give off heat (exothermic) or require heat to proceed (endothermic). More unusually, the can give off large amounts of energy in the form of light. This light is called chemiluminescence. In photochemistry, we are usually concerned with providing molecules with light to activate a reaction. With chemiluminescence, it’s the other way around – a chemical reaction results in the emission of light. The classic demonstration of chemiluminescence is with a compound called, appropriately enough, luminol. Here’s a short Youtube video on it (with a rather excited chemist).

So what is happening?

Let’s look closer at the luminol reaction. When hydrogen peroxide (e.g. from household bleach) is added to luminol, in the presence of base and a catalyst (such as iron(III) which gets involved in the oxidation), 3-aminophthalate is formed. But the energy involved in the oxidation of luminol by the peroxide results in the phthalate having an electronically excited state. The releases this excess energy by emission of light, giving the blue colour observed.

Luminol Reaction

Luminol reacts with hydrogen peroxide to produce an electronically excited 3-aminophthalate, which emits in the blue (450 nm)

Applications of chemiluminescence

Natural World

One of the most common observations of chemiluminescence, as any inhabitant of Pandora will know, is bio-chemiluminescence, or bioluminescence, which is where natural world has exploited the use of chemiluminescence. The most commonly known example of this is the firefly (Photinus), which uses a reaction similar to that of luminol, and an enzyme, luciferase, in place of the peroxide, along with magnesium ions to produce a glow (the colour depends on the type of fly).

Oxidation of Luciferin

Oxidation of Luciferin by luciferase in the presence of magnesium ions gives emission (e.g. in the yellow region)

As well as Pandora, back on Earth, Irish swimmers came across some beautiful examples of bioluminescence off the coast at Killiney when “spectacular green neon flashes” in the sea were observed by swimmers as they swam through water. This was determined to be the plankton Noctiluca scintillans, which is reported to be known as “Sea Ghost” or “Fire of Sea”.

Image of Noctiluca

Image of Noctiluca Scintillans (taken from Maria Antonia Sampayo, http://planktonnet.awi.de, Creative Commons Attribution 3.0 License)

Analytical Applications

Given that emission spectroscopy is such a versatile analytical tool, it is perhaps no surprise that chemiluminescence has several potential applications in the area of chemical analysis. The intensity of luminescence is proportional the the concentration of reactant. In principle, analysis does not need the same level of instrumentation as emission spectroscopy – which needs a light source to excite the sample and emission my be detectable by eye. Therefore it can be used in crude analytical tests. Luminol is used to test for blood at crimescenes – a luminol spray on any suspected blood traces results in the iron in the blood catalysing luminol chemiluminescence, and glows for up to a minute after being sprayed.

But there is more scope for its use. Two problems to its adoption as an analytical technique are that the quantum yield of emission can be low, which means that at low concentrations, the detection may be difficult. In addition to potentially poor sensitivity, long lived emission (such as those observed in glow sticks and luminol), which makes for great demonstrations, means that response time is unnecessarily long. Some work on both of these areas is advancing. Coupling the chemiluminescence interactions with metal nanoparticles harnesses the surface plasmon resonance effect, where the emission from the chemiluminescence couples or resonates with the electron density of the nanoparticles, which enhance the signal, with reports of a 4 – 10 fold increase. The efficient transfer of energy from the excited state of the reagent to the nanorparticles also significantly reduces their lifetime, hence the lingering glow. Readers interested in this work are referred to Aslan and Geddes, given below.

This being said, chemiluminescence is already in use to study a wide range of medicinal and environmental-related compounds in a technique that couples chemiluminescence with liquid chromatography (HPLC-CL). The reagents used in this technique include the now familiar luminol, which is used to investigate lipid hydroperoxides, neurotransmitters such as dopamine (which enhance the chemiluminescence), and environmentally relevant species such as organophosphorus reagents (e.g. diclorvos). The always familiar Ru(bpy)32+, which as well as everything else can exhibit chemiluminescence, undergoing reduction by analytes in high energy electron transfer reactions to produce the excited state. This system has been used to study nitrosamines, N-methylcarbamates pesticides in pear and apple samples and domoic acid which can be a factor responsible for shellfish poisoning. There are several set-ups possible for interfacing the chemiluminescence set-up with the HPLC; most simply by having an injection point after separation for the chemiluminescent reagent prior to the emission detector. A very detailed review of the use of chemiluminescence in medicinal, food and environmental analytes is that by Gámiz-Gracia et al.

Finally, it is worth noting that a lot of gas-phase reactions result in chemiluminescence. This is the basis of gas analysers, for example the nitrogen monoxide analyser. Nitrogen monoxide reacts with ozone to produce an excited state nitrogen dioxide which emits in the far visible/infrared region. The extent of luminescence can be related tot he initial concentration of NO.

Your very own magical world

If you want to set up your own version of Pandora, which cost James Cameron $250M, you can do it for a few euros, by buying some glow sticks and dotting them around. Their glow lasts for a few hours, so all you need is a little imagination during this time…

References

Lights in Sea are Natural“, Irish Times, www.irishtimes.com, 18 October 2009

K. Aslan and C. D. Geddes, “Metal-enhanced chemiluminescence: advanced chemiluminescence concepts for 21st century“, Chem. Soc. Rev., 2009, 2556 – 2564.

Laura Gámiz-Gracia, Ana M. García-Campana, José F. Huertas-Pérez, Francisco J. Lara, “Chemiluminescence detection in liquid chromatography: Applications to clinical, pharmaceutical, environmental and food analysis—A review“, Anal. Chim. Acta, 2009, 640, 7 – 28.




Beautiful Photochemistry

I came across this nice blog recently and thought it was worth signposting here. It is called “Beautiful Photochemistry” and its author writes summaries  of recent articles from some leading chemistry journals which have a photochemical basis. There are some great synopses on a range of topics within photochemistry, including one I was very happy to see on enone-alkene cycloadditions.

Beautiful Photochemistry Blog: http://beautifulphotochemistry.wordpress.com/

 

Our Energy Future: Lecture by Prof Tom Meyer

Prof Tom Meyer, Energy Frontier Research Centre, University of North Carolina, was in Dublin to participate in a Dublin Region Higher Education Alliance Master Class on Solar Energy. Afterwards, he gave a public lecture on “Our Energy Future: Science, Technology and Policy Challenges for the 21st Century – A US Perspective“. The lecture was held at TCD, and was sponsored by the Royal Society of Chemistry Republic of Ireland Local Section. It considered the various current and future world energy demands, and the role renewable energies have to play in providing this energy. My summary is given below.

Prof Thomas J Meyer has been researching the photochemistry of ruthenium complexes since the late 1960’s. Much of what we know about electron transfer in ruthenium polypyridyl complexes today is due to work conducted by Meyer and others in this period. Meyer worked with Henry Taube, who won the Nobel Prize in 1983 “for his work on the mechanisms of electron transfer reactions, especially in metal complexes”, publishing a paper with him in Inorganic Chemistry (1968) on excited state oxidation potentials of ruthenium-amine complexes. This work was an important pre-cursor to a 1973 paper published by Taube, Meyer and co-workers on the reduction of oxygen by these complexes. In the mid-1970’s, at a time when the oil crisis of the time was reaching a peak, Meyer published a series of important papers in Journal of American Chemical Society on the nature and kinetics of quenching of ruthenium amine complexes (including ruthenium – tris-bipyridyl) which gave great kinetic and mechanistic insight into the electron transfer between the metal complexes and an array of quenchers. Meyer reiterated in an article written in 1975 the importance of understanding electron transfer in the study of energy conversion, especially so with metal complexes as these absorb strongly at wavelengths of solar interest.

Prof Meyer, speaking at TCD on "Our Energy Future"

A surge of interest in these systems was observed the oil crisis, which faded somewhat in the 80’s and it wasn’t until Gratzel’s work on dye-sensitised solar cells, reported in 1990, that generated efficiencies that would allow for devices to become realistic contributors to energy supply. Since that itme, work has been concentrating on enhancing light absorption capacity, currently champoined by a ruthrnium dy “N3” (see DSSC post), as well as considering and optimising electron transfer processes in the solar cell devices.

Meyer’s lecture in TCD considered the current and future status of energy demands. It was a message he has delivered to the american political system, across administrations, during his tenure at the Los Alamos National Laboratory. Meyer reported that in the US, energy costs make up 7 – 10% of the cost of living, and 7% of overall world trade. A large demand in energy increase has been observed since 1900’s and this surge is expected to continue until at least 2100. While current stable economies’ energy usage will level off, emerging and transitional ecomomies (China, India, etc) will place major demands on the world’s energy supply. In the six years since 1999, China and India increased their energy usage by 80% and 25%, respectively (Cicerone). (A presentation by Cicerone, Preseident of the National Academy of Sciences is reference below and places thes enegy demands in context). In summation, >100 TW of additional ‘clean’ energy will be required by 2100.

The US currently uses 26% of the world’s oil supply, greater than the next five net using countries combined. 26% of the world’s oil is in the middle-east. Globally, the cost of oil is increasingly expensive to extract, as reserves become more and more difficult to source. Therefore additional energies from alternate sources is required to factor the loss in and increasing expensive of oil production; as well as the surge in energy demand from emerging economies. In addition, this energy supply must be in the context of envrironmental considerations, primarily global warming.

Meyer outlined several strategies to large scale energy production. Principal among these were nuclear, solar, and clean hydrocarbons. These and others are considered below.

Coal currently supplies 27% of the world’s energy demands, including half of US energy needs. It is also responsible for 35% of US carbon dioxide emissions. In principle, it could provide increased energy requirements until 2050, if 1% of GDP was used in dealing with carbon dioxide sequestration. The story of coal usage inclues the story of FutureGen – an initiative announced by the Bush administration in 2003. This was aimed at using coal as a clean fuel, with achieved targets of 275 MW of energy production with 90% carbon dioxide sequestration. However, the project was cancelled by the Bush administration in Jan 2008, due to massive cost overruns ($900M). The Obama adminsitration has restarted this work (June 2009), recognising that clean coal will be a crucial element to supplying energy demands in the forseeable future. Oil shale and tar sands are estimated to contain 2 trillion barrels of oil. However, it expensive (requireing a lot ofwater) and enviornmentally damaging to extract oil from these reserves.

Hyrdogen fuel is obtained from a variety of sources – primarily methane, but also from coal extraction and water electrolysis. In the latter case, electrolysis of water to produce hydrogen (and oxygen) is utililised by photochemical processes. Meyer identified the Idaho National Laboratory hydrogen programme as one which was making good progress in the production of hydrogen as a mass fuel. The advantages of hydrogen were good efficiency, and water and heat as emission products. However, the current costs (for transportation) are ca. $3500/kW, with a target of $35/kW. Another significant problem with the use of hydrogen was storage and transportation, which were expensive because of the nature of the fuel.

Nuclear energy provids ~20% of US energy, and increased usage would result in a significant decrease in greenhouse gases. There are 44 nuclear reactors currently being built internationally, and therefore these will be significnat contributors in to the future. The issues, well know, of nuclear power are what to do with waster, control (political issue), reprocessing and general safety issues.

Renewable energies provide an alternative approach to the solution. It is estimated that wind could provide 20% of US energy requirements. However, solar energy is a real viable option, given that 26,000TW per year of sunclight isiincident on the Earth’s surface (net amount after absorption etc). the technology is on the cusp of mass implementation, with some lingering problems regarding efficiencies. (In the US, there are also problems regardingthe arrangement of the national grid (see Grid 2030 project). Current estimates are that solar generation of 3 TW, assuming 10% efficiency solar cells, would cost approximately $60 Trillion (covering an area of 57k sq – miles). Current and future work will be focussed on reducing this cost.

Meyer reiterated the point in his talk, and again in questions, that there must be a political will to drive this work forward. Solar energy could have emerged as a major player much earlier, if work started after the oil crisis had continued apace. 6% of US energy is currently sourced from renewable sources; with 85% from coal, oil and gas. The hope is that by 2059, these numbers can be reversed!

References

C. R. Bock, T. J. Meyer, D. G. Whitten, Photochemistry of transition metal complexes. Mechanism and efficiency of energy conversion by electron-transfer quenching, J. Amer. Chem. Soc., 1975, 97, 2909 – 2911.

R. J. Cicerone, National Academy of Sciences, Address to the 145th Annual Meeting, available at: http://www.nationalacademies.org/includes/NASmembers2008.PDF [Oct 2009]

Las Alamos National Lab: National Security Science: http://www.lanl.gov/ [Oct 2009]

T. J. Meyer and H. Taube, Electron transfer reactions of ruthenium ammines, Inorg. Chem., 1968, 7, 2369 – 2371.

J. R. Pladziew, T. J. Meyer, J. A. Broomhea, and H. Taube, Reduction of oxygen by hexamammineruthenium(II) and by tris (ethylenediamine) ruthenium (II), Inorg. Chem., 1973, 12, 639 – 643.

H. Taube, Nobel Prize Lecture Nobel Prize 1983, http://nobelprize.org/nobel_prizes/chemistry/laureates/1983/taube-lecture.html [Oct 09]

R. C. Young, T. J. Meyer and D. G. Whitten, Kinetic relaxation measurement of rapid electron-transfer reactions by flash photlysis – conversion of light energy into chemical energy using Ru(bpy)3(3+)-Ru(bpy)3(2+*) couple, J. Amer. Chem. Soc., 1975, 97, 4781 – 4782.

Metal Oxide Photocatalysis

Metal oxide photocatalysis is based on the use of metal oxides (for example titanium dioxide) as light-activated catalysts in the destruction of organic and inorganic materials and in organic chemistry synthesis. In this article, we will be looking at the use of thee types of materials in the degradation of organic matter, which has applicability in environmental remediation (aqueous and air-borne) and self-cleaning surfaces. The technique is already widely used in commercial applications, but is still hampered by one significant limitation. These materials generally absorb primarily ultra-violet light, and research in recent years has been concentrating on developing visible-light active materials, with an emphasis on nano-particulate materials to maximize surface area. This article discusses the background to metal oxide photocatalysis, using titanium dioxide as the exemplar material, and looks at strategies being researched to enhance the photocatalytic efficiency.

Introduction

Titanium dioxide is a white powder, with titanium in oxidation state IV. Its d-electron configuration is therefore d0, and the white colour is explained by the lack of d-d or metal centred transitions. It exists in several polymorphs – two of interest here: anatase and rutile. As it is a semiconductor, its HOMO is termed a valence band and LUMO is termed a conduction band. Light absorption effectively results in a ligand to metal charge transfer, electrons from oxygen are transferred to the vacant titanium d-orbitals. For anatase (3.2 eV) and rutile (3.0 eV), this transition is in the UVA region, resulting in a sharp absorption band at 390 – 400 nm.

Looking more closely at the electronic processes, promotion of an electron to the conduction band, on irradiation by UV light, results in a ‘hole’ in the valence band – essentially a detriment of the electron density that was localised on that orbital, and usually assigned a positive charge to symbolize the loss of negative electron (of course negative and positive are just arbitrary notations). The hole is powerfully oxidizing – the orbital very much wants to retrieve electron density just lost after light irradiation. It can retrieve this simply by the electron in the conduction band recombining with the valence band – recombination is a sum of radiative (i.e. emission may be observed) and non-radiative processes. Based on the energy gap law, the fact that rutile energy levels are closer mean that the non-radiative process is more efficient, and hence recombination is more efficient. This is an important observation which we will return to shortly.

Alternative pathways to recombination are possible, and as you can guess, these result in the use of these materials as photocatalysts. The hole has the potential to oxidise water that may be on the surface of the material resulting in the formation of hydoxyl radicals. Hydroxyl radicals are themselves very powerful oxidisers, and can easily oxidise any organic species that happens to be nearby, ultimately to carbon dioxide and water. Meanwhile, upstairs in the conduction band, the electron has no hole to recombine with, since it has oxidised surface bound water. It quickly looks for an alternative to reduce, and rapidly reduces oxygen to form the superoxide anion. This can subsequently react with water to form, again, the hydroxyl radical. The processes are summarized below.

Top: Light of energy exceeding band gap results in charge separation, with electron reducing a donor (usually oxygen) and hole oxidising a donor (usually water); bottom: summary of processes occuring

Top: Light of energy exceeding band gap results in charge separation, with electron reducing a donor (usually oxygen) and hole oxidising a donor (usually water); bottom: summary of processes occuring. Image based on Bahnemann (2004).

At the level of the material’s surface, the requirements for efficient photocatalysis can be deduced from the electronic reactions – there should be surface bound water to allow for efficient oxidation; and the water should be aerated to provide oxygen to the solution. Additionally, the degradation of the pollutant by the catalyst requires for the pollutant to be adsorbed or very close to the surface of the material, and hence the greater the surface area of the material, the more pollutant can adsorb. Nanoparticulate materials are therefore preferred as they vastly increase the surface area (see DSSC post).

Pilkington self-cleaning glass is an example of use of this technology in a commercial application. A thin film of nanoparticulate titanium dioxide is coated onto panes of glass (it is so thin that it is transparent). The glass is in the normal course of events, acquiring dirt. The titanium dioxide on the glass, once exposed to sunlight, produces hydroxyl radicals which degrade any surface adsorbed dirt. Once washed down with rain, this decomposed dirt is removed and the glass is ready for another cycle. The same process is observed for any organic species – they react with the hydroxyl radical to ultimately form carbon dioxide and water.

Given that the materials work readily, it is a good time to detail the limitations. the primary limitation is that the materials absorb only UV light, so the activation by sunlight is completed by the 5% of sunlight that is in the UV region. A large amount of research has looked into ways to enhance the visible light activity of the materials. Another limitation is the fact that recombination is an efficient, competitive process, and given that this is a less efficient process with anatase, it is generally accepted that anatase is a preferred photocatalyst to rutile. Below, we will discuss approaches taken to both increase the visible light absorption capability and increase the efficiency of subsequent reactivity over the recombination process.

Moving to Visible Light Absorption Capability

Given the requirement for UV light activation of TiO2, researchers became interested in tuning the materials so that they would become activated by visible light (e.g. room light) for applications for indoor use or by solar light for outdoor use. Various approaches were considered, and in 2001, a Japanese chemist named Asahi working out of Toyota labs, published a paper in the journal Science on nitrogen doped titanium dioxide materials. Nitrogen doping produced what is commonly called yellow TiO2 (because of, unsurprisingly, its yellow colour!) which showed effective UV and visible light activity. While there is some debate around how the activity is increased, the N-doped TiO2 is shown to have a much greater absorbance in the visible region (extending from a sharp cut off at about 390 nm to a broad cut off at above 500 nm). This subsequently increased the amount of visible light activity the material could absorb, and hence meant that visible light-activated photocatalysis was achievable.

There has been some discussion in the literature on the mechanism on enhancement of nitrogen doping, and the mechanism described here is one put forward by Nakoto (2004) and Irie (2003), and counters Asahi’s original explanation that the N-doping narrowed the gap between the valence band and conduction band of titania. these researchers proposed that the introduction of nitrogen introduced new occupied (i.e. electron rich) orbitals in between the valence band (which are comprised primarily of O-2p orbitals) and conduction band (which are comprised primarily of Ti-3d orbitals). These N-2p orbitals acted as a step up for the electrons in the O-2p orbital, which once populated had now a much smaller jump to make to be promoted into the conduction band.Once this process occurs, electrons from the original valence band can migrate into the mid-band gap energy level, leaving a hole in the valence band, which reacts as described before.

N-doping as explained by Nakoto and Irie. Doping with nitrogen results in a mid-band gap energy level which reduces the energy gap required for charge separation

N-doping as explained by Nakoto and Irie. Doping with nitrogen results in a mid-band gap energy level which reduces the energy gap required for charge separation

Increasing efficiency by incorporation of metal nanoparticles

Given that charge separation requires a great deal of effort, a second theme of research (as well as increasing visible light activity) is to facilitate charge separation. One clever way of doing this is to incorporate noble metal nanoparticles such as silver or gold into the titanium dioxide material. As an example, incorporation of a small amount of silver (1 – 5%) results in increased efficiency in photocatalysis. Silver has a “Fermi level” or electron accepting region at an energy just below the conduction band. Therefore, after light absorption and charge separation, the electron in the conduction band can be effectively trapped by the silver, while the hole oxidises water and forms hydroxyl radicals, without the threat of recombination. Various researchers, including our own work, have shown that there is an optimum amount or “Goldilock’s zone” of silver to add – just enough is needed so that there are silver sites dispersed through the material to rapidly trap electrons, but that too much silver may cover the titanium dioxide and prevent light absorption. In addition, too much silver may mean that the silver acts as a recombination site itself – essentially it will form a bridge between an electron and a hole.

The emission of titanium dioxide (and of similar studies with zinc oxide) can be interpreted as a measure of the recombination efficiency. Studies examining the emission of these metal oxides have demonstrated that the emission intensity reduces on increasing amounts of silver – indicating that the silver is trapping electrons and reducing electron-hole recombination, as indicated in the diagram below.

Incorporation of silver nanoparticles facilitate longer charge separation by trapping photogenerated electrons

Incorporation of silver nanoparticles facilitate longer charge separation by trapping photogenerated electrons

Heterojunctions

A similar strategy to that described above, an a rapidly evolving area, is the idea of incorporating different semiconductors which have different conduction band energy levels. The strategy is as before, trap the electron so the hole has more time to react. A simple example is the anatase-rutile heterojunction. Rutile has a smaller band gap (by about 0.2 eV) to anatase, although their valence band levels are at similar energies. Therefore, in an analogous fashion to the situation with silver, above, charge separation in anatase, followed by electron injection into the rutile conduction band means that there is a hole in the valence band of anatase that can freely oxidise water. It is no coincidence that the industry standard photocatalyst, Degussa P25, has a 75:25 ratio of anatase:rutile (it also has a very small particle size).

Summary

Semiconductor photocatalysis is the utilisation of photogenerated strongly oxidising hydroxyl radicals, which can be applied to a wide range of scenarios, including organic degradation (for pollution remediation) and in organic synthesis. Light induced charge separation, followed by generation of hydroxyl radicals is in the normal course of event reliant on UV light, given the energy gap (band gap) of titanium dioxide. Strategies to enhance the photocatalytic activity include doping to reduce the energy required for charge separation and incorporation of nanoparticles to lengthen the period of charge separation. The size of the materials is also a factor, as for degradation of materials, the pollutant needs to be very near to or adsorbed onto the surface of the semiconductor, and nanoparticulate materials mean that a greater surface area can be exploited.

References

Asahi, R., Morikawa, T., Ohwaki, T., Aoki, K. and Taga, Y., Visible-light photocatalysis in nitrogen-doped titanium oxides, Science, 2001, 294, 269 – 271. Asahi’s paper describing his results on N-TiO2. the work shows irradiation by UV-only and visible-only light, showing the enhancement by N-TiO2 with visible light source.

Bahnemann, D., Photocatalytic water treatment: solar energy applications, Solar Energy, 2004, 77, 445–459. Prof Bahnemann is one of Europe’s most active researchers in this field, and this very readable paper shows how the technology can and is used in solar decontamination technology.

Nakamura R, Tanaka T, and Nakato Y., Mechanism for visible light responses in anodic photocurrents at N-doped TiO2 film electrodes, J. Phys Chem. B., 2004, 108, 10617 – 10620. (See also Irie, H et al, J. Phys Chem. B., 2003, 107, 5483 – 5486). Papers explaining the origin of the hypothesis for the mid-gap energy levels introduced by nitrogen doping.

Seery, M. K., George, R., Floris, P. and Pillai, S. C., Silver doped titanium dioxide nanomaterials for enhanced visible light photocatalysis, J. Photochem. Photobiol A: Chemistry, 2007, 189(2-3), 258 – 263 and Georgekutty, R., Seery, M. K. and Pillai, S. C., A Highly Efficient Ag-ZnO Photocatalyst: Synthesis, Properties and Mechanism, J. Phys. Chem. C, 2008, 112(35), 13563 – 13570. these papers detail the incorporation of silver into titanium and zinc oxides respectively, including some consideration of mechanism.

Luminescent PET Sensors

PET sensors are very simple in principle and can signal the presence or absence of a very small amount of analyte in solutions/biological samples by using a readily available instrument – the fluorimeter. This article looks at the design principles of PET sensors, as well as examining how they may be modified to enhance specificity/selectivity. [Aug 2009]

1. Introduction

Luminescent sensors provide for an easy, often visual, method for detection of a wide range of ions, physical properties such as pH and other components such as nanoparticles. Their wide variety of sensing capabilities are already used in commercial devices, and a greater understanding of their mechanism of operation is leading to newer exciting concepts, such as molecular logic gates for computing. This article aims to summarise the background theory to luminescent sensors – especially concentrating on photoinduced electron transfer (PET) which we can consider as the third type of quenching (Type I and Type 2 discussed elsewhere – links will be here) – and examining practical applications in the context of this theory. In summarising the work here, special attention is paid to the work of Prof A. P. de Silva who is based in Queen’s University, Belfast and who is one of the pioneers in the area. Interested students are referred to some of his papers in the references, where even in the formal constraints of academic journals, his passion for the subject is very evident.

Here’s a short podcast outlining what we will cover in this section (with audio)
Vodpod videos no longer available.

2. Background

We’ve seen several times that molecules that have absorbed light may transfer their electron to another system – which is usually called a quencher, although is not constrained to this (see for example dye-sensitized solar cells). Because this electron is transferred after absorption of light by the molecule, the process is called photo-induced electron transfer (PET). It’s one of the great applications of photochemistry in general – excited states are better oxidisers and reducers than their ground state counterparts. Suppose we have donor (reducing agent, itself going to be oxidised) and acceptor (oxidising agent, itself going to be reduced) molecules as shown below, where the energy level of the donor HOMO is lower than the energy of the acceptor LUMO. As it stands, the donor cannot reduce the acceptor, as electron transfer is not energetically feasible. However, if we form a donor excited state, D*, by light excitation, the process becomes energetically feasible once the donor LUMO is higher than the acceptor LUMO. You should consider how excited states are also better oxidising agents.

Excited states are better reducing agents (and oxidising agents) than their corresponding ground states

Excited states are better reducing agents (and oxidising agents) than their corresponding ground states

In applying this general concept to this section, we will be looking at molecules that have both these components in one molecule. In this system, we will term them luminophore and receptor, which are analogous to the discussion of donor and acceptor respectively. The luminophore is the light absorber which results in an excited state and may luminesce, or accept an electron into the vacancy in the ground state, or donate an electron from the excited state. The receptor will accept/donate an electron from/to the luminophore. The two components are linked by a non-conjugated bridge (e.g. two sp3 carbon bridge). You may wish to consider why a non-conjugated bridge is required. The three components – luminophore-spacer-receptor – make up what we will term here a class III type of quenching; where the PET process occurs within one molecule.

Schematic of a Fluorophore-Spacer-Receptor model

Schematic of a Fluorophore-Spacer-Receptor model

The power of the model is that if the receptor’s energy level’s can be tweaked in the presence or absence of an analyte – for example an metal cation – and the energy levels between the receptor and luminophore are close, well the presence of the ion may turn on or shut off the electron transfer process. This concept is now a sensor, as luminescence is affected by the presence or absence of an analyte. If that luminescence can be measured (which it can), we have a very powerful, sensitive analytical tool to measure the extent of analyte present.

3. Details

3.1 Principles

The general principle discussed above is summarised in the diagram below. A luminophore gives off light in the absence of an analyte, but emission is quenched in the presence of an analyte. This is called an ON-OFF system – emission was turned off by the presence of an analyte. The systems become very powerful when the receptor – the docking area for the analyte – is selective to only a particular type of species (e.g. a halide) or even better a specific ion. An example of one of de Silva’s systems is shown below. The three components are obvious: the anthracene molecule is the luminophore (in this case a fluorophore as it emits from the excited singlet state); the bipyridyl component is the receptor and the two carbon-chain acts as a spacer. In the absence of an ion, this system fluoresces strongly. However, in the presence of either zinc ion of a proton the emission is shut off. So how can we explain what happened?

An example of an ON-OFF sensor: in the absence of protons or zinc ions, emission is observed (ON), whereas in the presence of either of these ions, emission is strongly quenched. (Based on de Silva, J. Am. Chem. Soc., 1999, 121, 1393)

An example of an ON-OFF sensor: in the absence of protons or zinc ions, emission is observed (ON), whereas in the presence of either of these ions, emission is strongly quenched. (Based on de Silva, J. Am. Chem. Soc., 1999, 121, 1393)

Examining the emission spectrum, we can see that the anthracene emission is very much reduced in the presence of one of the specific ions, but the shape or position of the bands do not change. Therefore its energy levels remain as they were prior to the presence of the receptor. It can be concluded that the receptor levels change, opening up a PET pathway that was not available prior to the presence of the analyte. The diagram below summarises the schematic, molecular and energetic processes.

Schematic (top); energy level (middle) and molecular (bottom) representations of an ON-OFF sensor. The dashed line in the energy level representation of the acceptor-analyte shows the energy level prior to binding of the analyte

Schematic (top); energy level (middle) and molecular (bottom) representations of an ON-OFF sensor. The dashed line in the energy level representation of the acceptor-analyte shows the energy level prior to binding of the analyte

3.2 Developing the idea further

In a paper discussing the developments of luminescent sensors so far this century, de Silva illustrates a scenario that is already a reality (Tetrahedron, 2005, 61, 8551 – 8588). An ambulance is called for a car accident trauma, and upon arrival paramedics take a blood sample. From this, they can alert the hospital en route of the required electrolyte levels required for a blood bag, having used a small portable instrument to determine concentrations of for example, sodium, potassium, calcium as well as oxygen levels and pH. The key here is that the the device incorporates a range of PET sensors, each of which have receptor components of the moleculethat selectively sense a particular of ion/molecule. Therefore, the question is how is this done?

Selectivity has been achieved by a range of interesting approaches, and we have to be thankful to organic chemists for their synthesis of the diverse range of molecules available. Crown ethers are useful receptors, and are known to bind well to sodium ions. The image below is from de Silva’s homepage. It shows the use of a crown ether in a PET sensor that selectively senses for sodium. The principle is the same as above (except this is now an OFF-ON sensor – you should sketch out the energy levels to see how emission is turned on in the presence of sodium ions)

Fluorescent sensor for sodium, with emission spectra showing an incease in emission on additional increments of sodium concentration (From AP de Silva, used with permission)

Fluorescent sensor for sodium, with emission spectra showing an incease in emission on additional increments of sodium ion concentration (From AP de Silva website - link in text, used with permission)

Now that the general principle is understood, it is easy to get selective across a wide range of ions. By modifying the size of the crown ether, the sensor can lose its selectivity for sodium ions and move on to potassium ions (see image below). Various minor modifications can make it calcium selective. Very quicky, the range of ion selectivities can be acquired.

An example of a sensor that is potassium ion selective (because of the larger crown ether diameter compared to the sodium ion equivalent, above), from de Silva et al, Dalton Transations - used by permission of the Royal Society of Chemistry - see link below)

An example of a sensor that is potassium ion selective (because of the larger crown ether diameter compared to the sodium ion equivalent, above), from de Silva et al, 2003, Dalton Transations - used by permission of the Royal Society of Chemistry - see link below)

The principle can also be applied as a pH monitor, by developing a proton sensitive detector. In this case, amines linked to an anthracene molecule via a spacer make for a very simple but effective pH sensor. An example is shown below – a clever attribute of this design is that the use of two amines mean that much greater sensitivity across the range of pH is “built in” to the molecules design. The emission at various pH values is shown alongside the molecule. You should aim to sketch out the appropriate changes to the molecule on increasing acidity resulting in this change in emission. As well as simple pH monitors, these types of materials have been used in examining the effectiveness of cancer treatment. Cancer cells respond to treatment by developing acidic environments, and by examining the emission of a pH sensor on treating with radiation. The images, available in fig. 3 in the original paper show increased pH, easily detected, indicating that treatment is having an effect.

An amine-anthracene pH sensor, with changes in emission in at different pH's shown. (Source unknown)

An amine-anthracene pH sensor, with changes in emission in at different pH's shown. (Source unknown)

As well as cations, a lot of work has been invested into developing anion sensors. Gunnlaugsson, based in Trinity College Dublin is a leading researcher in this area and a suummary of many of his results can be found in the reference at the end. A video demonstrating the effect will be posted here September 09.

4. Conclusion

The principle of photo-induced electron transfer can be utilised in the development of molecular sensors. The mechanism of PET was examined above, as well as looking how shutting off or turning on this process provides information on the nature of ions interacting with the sensor. Sensors can be designed so that they  selectively sense an ion in the presence of others. Current research in this area is looking at using these principles in molecular computing, and this work will be surveyed in a future article.

[Aug 2009 – updates/amendments will be logged in comments.]

5. References

Newer optical-based molecular devices from older coordination chemistry, de Silva, A. P., McCaughan, B., McKinney, B. O. F. and Querol, M, Dalton Trans., 2003, 1902 – 1913. Really excellent overview of the area with lots of examples.

Luminescent sensors and switches in the early 21st century, Callan, J. F., de Silva, A. P. and Magri, D. C., Tetrahedron, 61, 8551 – 8588. Overview of  developments in 21st Century across range of analyte types (cations, anions, molecules)

Anion recognition and sensing in organic and aqueous media using luminescent and colorimetric sensors, Gunnlaugsson, T., Glynn, M., Tocci (nee Hussey), G. M., Kruger, P. E. and Pfeffer, F. M., Coord. Chem. Rev. , 2006, 250, 3094–3117. Good overview of anion sensors.

Dye-Sensitized Solar Cells (DSSC)

Increasingly, solar energy has a vital role to play in providing energy to cater for an ever increasing demand. This article looks at what dye-sensitised solar cells are and their current technological status, as well as what needs to be done to make them big hitters in the energy game. It summarises some recent reviews on the topic, interested students are pointed to the source material and other references at the end of the article. [Aug 2009]

1. Introduction

A FEW YEARS AGO when talking about dye-sensitised solar cells in lectures, a student asked me what the problem with them was. Gratzel had published his seminal paper in 1991, and now, over 15 years later, from an outside observer’s perspective, there wasn’t much progress in terms of developing a commercial applicable devide. It’s a good question, and one worth asking periodically of any innovation. We hear that drugs often take 10 – 15 years from bench to patient, so one might reasonably ask with DSSC – “What’s the delay?“.

Here’s a short podcast (with audio) introducing this section outlining what this article will cover:
Vodpod videos no longer available.

2. Context

Now that we are near the end of the first decade of the 20th century, future energy demands make for sobering reading. The world currently uses about 13 terawatts (TW) of energy, and it is predicted by 2050, an additional 10 TW will be required. Not only that, the additional energy required will be have to be carbon neutral. Not only that, we are heading for peak oil. Considering all of this, what is the role of solar energy?

Kamat (2007) has summarised the role of solar energy very clearly with data that… ahem… blows other alternative/renewable energy sources out of the… ahem… water. Of the 10 TW additional energy required, building 1 GW nuclear power station every day for the next 50 years would meet the demand. (Nuclear isn’t strictly speaking renewable.) Hydroelectric could provide about 0.5 TW, tides and oceans could chip in 2 TW and wind power could blow 2 – 4 TW our way. Solar energy striking the earth amounts to 120,000 TW, yet only 0.01 – 0.04% of current energy usage is derived from solar sources. (Approximately 13% of current energy needs being supplied by renewable sources.) In theory, one hour’s solar irradiation is enough to supply a year’s global energy demands.

How would increasing the role of solar energy manifest itself on the ground? Solar flux at ground level is approximately 340 W/m2 in the world’s sunniest areas. Assuming 10% efficiency, each metre-squared of solar cell could generate 34 W. Plugging in the numbers, you’d be looking at a mere 4 x 10^11 m2, or about 618 km square to address the increase in world energy needs by 2050. While large, it’s not unrealistic. (Dublin county has an area of approximately 115 km square). The map below shows how, based on average sunlight irradiance measured over 1993-1994, how placement of solar energy “stations” at various locations around the planet would provide a substantial amount of solar-derived energy.

Total Primary Energy Supply: Required Land AreaRequired land area to supply an average of 18 TW (by Matthias Loster, 2006, reproduced with permission, full details on source website)

3. Overview

Dye-sensitised solar cells are as a concept, ingeniously simple. The idea was first conveived in the late 1970’s but since a Swiss photochemistr, Michael Gratzel, published a Nature paper in 1991 reporting 7% efficiency, the interest in the systemhas grown enormously. The outline of a DSSC is shown below and discussed in detail in section 4. Light harvesters gather in energy from a solar/light source and pass on the energy to an electrical circuit which does work (how do you think this compares with nature’s way of generating energy from sunlight?!).

Schematic of a Dye-Sensitized Solar Cell (DSSC) showing energy levels and electronic transitions

Schematic of a Dye-Sensitized Solar Cell (DSSC) showing energy levels and electronic transitions

Light harvesting dyes absorb solar radiation incident on them. This results in excitation of these molecules, who pass the energy obtained by means of transferring electrons onto a nanocrystalline TiO2 substrate onto which they are adsorbed. The electrons, now in the conduction band of titanium dioxide, conduct around a circuit and do work. At some counter electrode, a redox couple is utilised (usually iodide-triiodide) to regenerate the dye so the process can occur all over again. Assuming the dye is efficient at harvesting light, the transfer of electrons to titanium dioxide is efficient, the conduction of electrons in the circuit displays good potential and the dye can regenerate multiple-million times before being degraded, the concept works well with reasonable efficiency (~7 – 11%).

Slideshow: Electronic Pathways in Dye-sensitised Solar Cells:

The chemistry involved is sandwiched between two sheets of conducting glass, coated with a conductive layer (e.g. ITO) which is transparent. One plate of glass (working electrode) is coated with titanium dioxide nanoparticles that have the dye adsorbed onto the surface and the other (counter electrode) is coated with a catalyst (platinum or carbon). The plates contain electrolyte solution between them with the redox couple to regenerate the dye.

gratzel cell schematic showing forward and reverse processes

Forward and reverse electronic pathways in a dye-sensitized solar cell

Here’s a video of a DSSC in action:

4. Developing the idea further

The efficiency of dyes can be measured by consdering how many absorbed photons result in electron injection and how many of these injected electrons are collected to be used in the electrical circuit. This is expressed according to Equation (1), where IPCE(λ) is the incident photon to current efficiency – a measure of how many photons translate into electrical current.

IPCE(λ) = LHE(λ) × Φ(inj) × η(c)

The light harvesting efficiency (LHE) is the fraction of photons absorbed by the dye at a particular wavelength. The electron injection efficiency (Φ(inj)) is a measure of how many absorbed photons result in an injected electron into the semi-conductor and the charge collection efficiency is a measure of how many of these injected electrons are collected for electrical use. The equation essentially maps out each of the processes in the cell and considers their efficiency. All of the processes in the DSSC are kinetic – their efficiency is determined by how fast they occur relative to competing processes. We’ll consider below the various components of the dye and identify where any efficiencies could be improved upon in future developments. This section is based mainly on Hupp (2008 and subsequent more recent articles).

4.1 Dye Characteristics

The light harvesting dye is clearly a crucial component of the cell design and needs to fulfil several criteria; adsorption onto metal surface, overlap effectively with solar spectrum, inject electrons efficiently into metal oxide and be stable for many million cycles.

Adsorbtion of the dye onto the metal oxide surface is facilitated by incorporating a substituent that will adsorb readily. The most efficient studied are ruthenium dyes with carboxyl-substituted ligands – these carboxyl substituents adsorb onto the dyes surface.

run3-tio2

Solar Spectrum and Ru-N3 dye (left-most red spectrum) Additional spectra are predicted overlap from changing dye energygap (Hupp el at, 2008 Reproduced by permission of the Royal Society of Chemistry - original article linked below)

Top - Ruthenium based "N3" dye adsorbed onto a titanium dioxide surface; Bottom: Solar Spectrum and Ru-N3 dye (left-most red spectrum) Additional spectra are predicted overlap from changing dye energygap (Hupp el at, 2008 Reproduced by permission of the Royal Society of Chemistry - original article linked below)

The spectral overlap with the solar spectrum should be maximised so that as much of the sun’s energy as possible is utilised in exciting the dye, and promoting a high density of electrons into the excited state. In practice, dyes absorb in the visible and near infrared region (about 400 – 700 nm), capturing about half the available power and a third of the available photons from solar source. The ruthenium complexes which are currently “best in show” do have a limitation in that their exctinction coefficients are comparatively low (1 – 2 x 10e4 M-1 cm-1), requiring several hundred monolayers which in turn requires the metal oxide support to have a very high surface area. To achieve efficiencies of >15%, DSSCs will need to absorb bout 80% of light between 350 – 900 nm. Therefore using materials that have a higher absorption capacity may be a useful future strategy. Research here has included using osmium in place of ruthenium, which extended the absorption further into the red and enhanced the response of the cell to light relative to the ruthenium analogue. The 1MLCT to 3MLCT transition in osmium is much more intense than in ruthenium. Organic dyes have also been used successfully as attested by the very many articles and school projects on using fruit berries as the dye in these cells. A range of dyes are shown below, and while they vary in chemical structure, you should note a common factor between them and between these and the ruthenium dye shown above. Organic dyes have much larger extinction coefficients (5 – 20 x 10^5 M-1 cm-1), albeit across a narrow range than the ruthenium counterparts. (It seems that dyes will either absorb moderately well across a broad range or very well across a narrow range!)

dssc_dyes

Organic dyes utilized in (a) 9% efficient indoline, (b) 6.5% efficient coumarin, (c) 5.2% efficient hemicyanine, (d) 4.5% efficient squarine, (e) 7.1% efficient porphyrin, and (f) 3.5% efficient phthalocyanine-based DSSCs. (From Hupp et al, Reproduced by permission of the Royal Society of Chemistry - Original Article linked below)

In order for electronic transfer to be energetically favourable, the excited state energy of the dye should be higher in energy than the conduction band of the semiconductor. As well as this, the kinetics of electron injection into titanium dioxide should be faster than recombination of the dye (by luminescence or non-radiateive decay). This isn’t a problem with the N3 dye, above which injects on a femtosecond timescale and decays at a much more leisurely sub-picosecond timescale. (You might consider in your studies how this data would be determined experimentally). Electrons are injected from both the 1MLCT (on a sub-ps timescale) and the 3MLCT, which is formed within 100 fs by intersystem crossing (due to the heavy atom effect of ruthenium). Recent research (Durrant, J. Am. Chem. Soc., 2009, 131, 4808) has questioned whether similar difference in rates are observed in real DSSC (as opposed to model systems), based on results showing that in these real systems, the electron injection slowed down to ps timescale, allowing recombination to be competitive. Nevertheless, it is considered that this process is efficient, although caution is required in ensuring that efficiency isn’t reduced by other design factors on the cell.

Timescales involved for electron injection

Timescales involved for electron injection from Ru-dye to TiO2

Considering the discussion above regarding extending the dye’s absorption further into the red, this could be achieved by lowering the LUMO of the dye, although researchers have been reluctant to do this as since the LUMO-CB transition is so fast, the energy levels must be very well matched. (Goldilocks Principle: Not too high, not too low, just right).

Very recent research at Stanford University has coupled luminescent chromophores which absorb high energy photons and pass their energy on to the sensitising dye (Nature Photonics, 2009, 3, 406 – 411) – I’ll put more on this in a future article.

4.2 Metal Oxide Support

Following sucessful injection into metal oxide, the next phase is for the elctron to percolate through the oxide layer onto the working electrode. TiO2 is the most common substrate used. As a chemical, it is relatively inert, cheap and can be synthesised via the sol-gel process (offering flexibility and scalability). Unlike silicon solar cels, very high purity is not required. It exists in various forms, mainly anatase, rutile and brookite. Anatase has a high band gap (3.2 eV compared to rutile’s 3.0 eV) which gives several advantages. It absorbs very little of the solar spectrum, meaning it is transparent to incoming light source (so that the dyes rather than the metal oxide is activated by light). In addition, the larger band gap than rutile means that recombination is slower (ref the energy gap law) – some reports have determined a 30% lower efficiency with rutile.As with the dye, there are several factors to consider to maximise efficiency.

Gratzel’s innovation in his 1991 paper was among other things to use nanocrystalline TiO2. The nanomaterial’s much greater surface area meant that many more molecules of dye could adsorb onto the surface and pushed efficiency to the best reported at the time: 7%. Nanocrystalline surfaces in these cells are reported to have a surface: geometrical area of ca. 1200. Best performers actually have two distinct layers of metal oxide: a 12 micron thick transparent layer of 10 – 20 nm sized particles covered with a 4 micron thick layer of much larger (400 nm) particles. The larger particles scatter photons back into the film.

One significant limitation of this model is the very slow time taken for electrons to percolate through the material and onto the transparent conducting electrode; which is of the order of 100’s of microseconds, in comparison with conduction band-dye recombination which is of the order of a couple of microseconds. (It is however faster than the other decay process: conduction band-redox couple, which is in the millisecond range). Because of this, researchers are looking at improving the dynamics of electron percolation through the film. One strategy is to use nanotubes/rods rather than particles. These have the disadvantage of having lower surface area, but an advantage of very much improved electron transport because they provide an physical electron pathway from the particle to the electrode. Pagliaro (2009) gives a nice summary of this and this and other work on ZnO nanorods will be summarised in a future article.

4.3 Electrolyte and Regeneration

The electrolye contins the redox couple which regenerates the oxidised dye whcih were formed byinjection of electron from the dye to the titanium dioxide layer, leaving D+. The redox couple has a difficult job – it must be very efficient at reducing the dye cation back to the original state for another cycle, but not intercept or capture one of the electrons being injected in the first place! The latter process, which would result in inefficiency involves transfer of an electron from the conduction band of titanium dioxide to the redox couple (to the triiodide ion I3-). Of the range of redox couples studied, iodide-triiodide has proved to be in the most efficient  cells, mainly because the transfer of electron from titania to trioidide is, as mentioned above, very slow (in the millisecond time range). One of the problems in trying to look for improved systems is that the redox chemistry of this redox couple is not well understood, so it is difficult to plan effective alternative dyes. One interesting approach is to use solid state redox couples, which allow for higher concentrations of the redox couple and extending the applicability of the device (because of removal of the liquid electrolyte).

5. Summary and Review

Dye-sensitized solar cells offer enormous potential as an alternative renewable energy provider. The principle of operation is for a light harverster to absorb light efficiently and pass on the energy to a metal oxide surface, which links in to a circuit generating current. The dye is regenerated at the counter-electrode via a redox couple. In reviewing dye sensitised solar cells, you should consider the two main themes covered in this article:

  1. Describe the principle of operation outlining the pathway the electron follow from light absorption to dye regeneration. Using the diagram below, represent these processes on an energy level diagram and for each of the stages outline the counter process available resulting in efficiency. By using time constants/scales, indicate whether these processes are likely to significantly impact on the efficiency of the cell.
  2. For each of the processes, outline improvement strategies that are being considered in current research in the area, indicating the rationale for such approaches.
Kinetic Processes in DSSC - you should consider the relative times for the various processes involved

Kinetic Processes in DSSC - you should consider the relative times for the forward(green) and reverse processes (purple arrows) involved

Addendum:

This diagram below, from Hupp et al., Chem. Eur. J., illustrates effectively the rates of competing processes:

Normalised rates of competitive processes in each stage of the DSSC. Note the "double hump" for charge injection - you should relate this to the schematic for charge injection above. (J. T. Hupp et al, New architechtures for dye-sensitized solar cells, Chem. Eur. J., 2008, 14, 4459. Copyright Wiley-VCH Verlag GmbH & Co. KGaA. Reproduced with permission.)

Normalised rates of competitive processes in each stage of the DSSC. Note the "double hump" for charge injection - you should relate this to the schematic for charge injection above. (J. T. Hupp et al, New architechtures for dye-sensitized solar cells, Chem. Eur. J., 2008, 14, 4458. Copyright Wiley-VCH Verlag GmbH & Co. KGaA. Reproduced with permission.)

Further Reading/References

  1. Meeting the clean energy demand: nanostructure architechtures for solar energy conversion, P. V. Kamat, J. Phys. Chem. C, 2007, 111, 2834 – 2860. Excellent article on the wide variety of roles nanotechnology may play in providing future energy needs. This paper was used to provide much of the context. Prof. Kamat’s website is also worth looking at. Google “Light Energy Conversion” and this is the web-page that comes up, with good reason.
  2. Advancing beyond current generation dye-sensitized solar cells, Hamann, T.W., Jensen, R. A., Martinson, A. B. F., Van Ryswyk, H and Hupp, J. T., Energy Environ. Sci., 2008, 1, 66 – 78. An excellent paper on the properties of DSSC and how they may be technically advanced in future developments. Section 4 is substantially based on this article. Link
  3. Dye-sensitized solar cells: a safe bet for the future, Gonçalves, L. M., de Zea Bermudez, V., Aguilar Ribeiro, H. and Magalhães Mendes, A, Energy Environ. Sci., 2008, 1, 655 – 667. A concise overview on the device aspect of DSSC, considering current and future implementation requirements.
  4. Nanochemistry aspects of titania in dye-sensitized solar cells, Pagliaro, M., Palmisano, G., Ciriminna, R. and Loddo, V., Energy Environ. Sci., 2009, 2, 838 – 844. Good overview of the photoanode requirements and research developments.
Now more than ever, solar energy has a vital role to play in providing energy to cater for an ever increasing demand. This post looks at what dye-sensitised solar cells are and their current technological status, as well as what needs to be done to make them big hitters in the energy game. It summarises some recent reviews on the topic, interested students are pointed to the source material and other references at the end of the article.

Published August 2009. Subsequent additions/amendments/corrections will be logged in comments.